Optical line testing device using optical signals having continuous waveform to identify fault location in optical line

ABSTRACT

An optical line testing device for measuring at least a cutting position of an optical line according to the present invention includes: a first wavelength tunable laser source configured to generate a first optical signal in which a plurality of wavelengths appear alternately and periodically; a second wavelength tunable laser source configured to generate a second optical signal which is identical to the first optical signal but has an adjustable delay time; and an interferometer configured to cause interference between a reflected optical signal, corresponding to the first optical signal, which is returning after having been emitted to the optical line, and the second optical signal to output an interference signal.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a continuation of U.S. application Ser. No. 15/539,267 filed onJun. 23, 2017, which is a National Stage of International ApplicationNo. PCT/KR2015/014061, filed Dec. 22, 2015, and claims priority fromKorean Patent Applications No. 10-2014-0188002 filed Dec. 24, 2014, thecontents of which are incorporated herein by reference in theirentireties.

BACKGROUND 1. Field

The present invention relates to an optical line testing device such asan Optical Time Domain Reflectometer (OTDR), and more particularly, toan optical line testing device using a wavelength tunable laser.

2. Description of the Related Art

As communication volume increases, copper-based communication cables arereplaced by optical fiber-based optical lines. The optical lines havebeen installed only in a section connecting a telephone office and atelephone office, but due to the increase in multimedia services such asVideo On Demand (VOD), the optical lines are now being installed inhomes or each room of the homes like a Fiber To The Home (FTTH).Therefore, as a service provider, management of numerous optical linesand detection of fault points have become very important in managementof a communication network.

One of devices for managing the optical lines is an optical line testingdevice, for example, an Optical Time Domain Reflectometer (OTDR). Asshown in FIG. 1, the OTDR generates an optical pulse 2 whose power ishigh and whose width is short in a laser 1, and enters the optical pulse2 into the optical line 3 to be tested to start a test. If there is afine-cut surface 4 somewhere in the optical line 3, the optical pulse 2here makes a reflection pulse opposite to the traveling direction,receives the reflection pulse again, and generally displays the resultlike FIG. 2. Since an operating principle of the OTDR corresponds to aknown technology, a detailed description thereof will not be givenherein.

(References: Korean Patent Publications No. 2004-23305 and No.1997-28648).

However, a classical OTDR using an optical pulse is a useful tool formanaging quality of an optical line in most cases, but has the followingdisadvantages.

First, it is difficult to increase a dynamic range. The dynamic rangerefers to a distance the OTDR can measure. To increase the range, it isnecessary to increase magnitude of the optical pulse. However, if themagnitude of the optical pulse is increased beyond a threshold value, anonlinear effect due to an interaction between the optical line and theoptical pulse is strongly generated, and a shape of the optical pulse isdistorted to cause a measurement error.

At present, to avoid such an error, a length (width) of the opticalpulse is increased instead of increasing the magnitude of the opticalpulse. This increases the dynamic range. However, as the length (width)of the optical pulse increases, a resolution of the OTDR deteriorates asshown in FIG. 3. The resolution is improved as the length of the opticalpulse is shorter. The resolution is represented by parameters such as anevent dead zone and an attenuation dead zone and all of the parametersare connected to each other, so that if one property is improved, theother property is damaged.

Furthermore, an Erbium Doped Fiber Amplifier (EDFA) may be used as analternative method for increasing the dynamic range. However, since aconventional method of the OTDR uses an optical pulse having extremevariations in optical power over time, it is inappropriate to use theEDFA for amplifying the optical pulse.

As described above, according to the prior art, there is a limit tofurther improve the dynamic range and the resolution, and therefore, atechnique capable of solving the problem is required.

SUMMARY

The present invention is directed to an optical line testing devicecapable of improving a dynamic range and a resolution.

Furthermore, the present invention is directed to an optical linetesting device capable of minimizing a nonlinear effect caused by anoptical line and using an optical amplifier such as an Erbium DopedFiber Amplifier (EDFA).

According to an aspect of the present invention, an optical line testingdevice for measuring at least a cutting position of an optical line, theoptical line testing device includes:

a first wavelength tunable laser source configured to generate a firstoptical signal in which a plurality of wavelengths appear alternatelyand periodically (hereinafter, a period in which one wavelength appearsrepeatedly is referred to as a ‘wavelength repetition period’); a secondwavelength tunable laser source configured to generate a second opticalsignal which is identical to the first optical signal but has anadjustable delay time; and an interferometer configured to causeinterference between a reflected optical signal, corresponding to thefirst optical signal, which is returning after having been emitted tothe optical line, and the second optical signal to output aninterference signal, and the optical line testing is configured tomeasure the output of the interference signal while varying the delaytime.

According to another aspect of the present invention, an optical linetesting device for measuring at least a cutting position of an opticalline, the optical line testing device includes:

a first wavelength tunable laser source controlled by a first wavelengthcontrol signal to generate a first optical signal in which a pluralityof wavelengths appear alternately and periodically (hereinafter, aperiod in which one wavelength appears repeatedly is referred to as a‘wavelength repetition period’); a second wavelength tunable lasersource controlled by a second wavelength control signal which isidentical to the first wavelength control signal but has an adjustabledelay time to generate a second optical signal; and an interferometerconfigured to output an interference signal by causing interferencebetween a reflected optical signal, corresponding to the first opticalsignal, which is returning after having been emitted to the opticalline, and the second optical signal, and the optical line testing isconfigured to measure the output of the interference signal whilevarying the delay time.

The optical line testing device may further include a delay unitconfigured to output the second wavelength control signal by delayingthe first wavelength control signal by the delay time.

The optical line testing device may configured to measure the output ofthe interference signal while varying the delay time.

The optical line testing device may be configured to calculate thecutting position by using a delay time at which the measured outputbecomes maximum.

The optical line testing device may be configured to measure an outputof the interference signal while varying the delay time for two or moredifferent wavelength repetition periods, and in all of the two or moredifferent wavelength repetition periods, calculate the cutting positionusing a delay time at which the measured output becomes maximum.

The optical power of the first optical signal and the second opticalsignal may be constant or continuous.

The interferometer may include a polarization controller for matchingpolarizations of the reflected optical signal and the second opticalsignal.

The optical line testing device may further include an optical signalreceiver configured to convert an optical signal output from theinterferometer into an electrical signal.

According to still another aspect of the present invention, an opticalline testing device for measuring at least a cutting position of anoptical line, the optical line testing device includes:

two wavelength tunable laser sources configured to each generate anoptical signal in which a plurality of wavelengths appear alternatelyand periodically (hereinafter, a period in which one wavelength appearsrepeatedly is referred to as a ‘wavelength repetition period’), whereina first optical signal generated from one of the two wavelength tunablelaser sources is emitted to the optical line,

a variable delay time is given to a second optical signal generated fromthe other one of the two wavelength tunable laser sources, and theoptical line testing device is configured to measure the cuttingposition using an interference effect when the round trip time,corresponding to the first optical signal, which is returning afterhaving been emitted to the optical line, is equal to or an integralmultiple of the delay time of the second optical signal, and isconfigured to measure an output of an interference signal while varyingthe delay time.

According to an embodiment of the present invention, unlike an opticalpulse, optical power of a used optical signal is constant or continuous,so that even if the optical power increases, a nonlinear effect betweenthe optical signal and an optical line does not occur or decreases, andthus optical power of an optical signal for measurement may beincreased. Therefore, a dynamic range may be greatly improved withoutsacrificing a resolution of an optical line testing device.

According to an embodiment of the present invention, it is possible toprovide an optical line testing device with excellent resolution anddynamic range, and to precisely measure a cutting position or the likeon a long-distance optical line.

Furthermore, according to an embodiment of the present invention, it ispossible to minimize a nonlinear effect caused by a measured opticalsignal in an optical line so that an optical amplifier such as an ErbiumDoped Fiber Amplifier (EDFA) may be used.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a view for explaining a measurement principle of a generalOTDR;

FIG. 2 is a graph of a signal waveform showing a measurement result of ageneral OTDR;

FIG. 3 is a view for explaining a relationship between an optical pulsewidth and a resolution;

FIG. 4 is a block diagram of an optical line testing device according toan embodiment of the present invention;

FIG. 5 is a view of a polymer wavelength tunable laser;

FIG. 6A is a view of a relationship between a wavelength control signaland an optical signal output from a wavelength tunable laser sourceaccording to the wavelength control signal, FIG. 6B is a view of arelationship between two wavelength control signals when an output of anoptical signal receiver is maximum, and FIG. 6C is a view of arelationship between two wavelength control signals when an output of anoptical signal receiver is minimum;

FIG. 7A is a graph of an output of an optical signal receiver accordingto a delay time when it is assumed that a length of an optical line tobe measured is zero and a length of all internal optical lines in anoptical line testing device is also zero, and FIG. 7B is a graph of anoutput of an optical signal receiver according to a delay time whenthere is a length difference in a path of an optical line;

FIG. 8A is a view for explaining that a maximum point in an output of anoptical signal receiver is not clearly distinguished, and FIG. 8B is aview for explaining a situation of using a wavelength control signalhaving a plurality of different periods; and

FIGS. 9A to 9C are views of a plurality of wavelength control signalshaving different periods and an output of a wavelength tunable lasersource according to each of the plurality of wavelength control signals.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

Features and advantages of the present invention will become apparentfrom the following detailed description of the present invention withreference to the accompanying drawings, and it will be understood bythose of ordinary skill in the art that various changes in form anddetails may be made therein without departing from the spirit and scopeof the present invention. Furthermore, in the description of the presentinvention, certain detailed explanations of the related art are omittedwhen it is deemed that they may unnecessarily obscure the essence of thepresent invention. Reference will now be made in detail to embodimentsof the present invention, examples of which are illustrated in theaccompanying drawings.

FIG. 4 is a block diagram of an optical line testing device 100according to an embodiment of the present invention.

The optical line testing device 100 according to an embodiment of thepresent invention is for measuring at least a cutting position of anoptical line and includes a first wavelength tunable laser source TOS1,a second wavelength tunable laser source TOS2, a delay unit DL, a firstdirectional coupler DC1, an optical signal receiver PD, ananalog-to-digital converter ADC, a signal processing and controllingunit CONT, and an interferometer IFM.

The first and second wavelength tunable laser sources TOS1 and TOS2receive a wavelength control signal from the signal processing andcontrolling unit CONT and output light of a wavelength correspondingthereto.

The first and second wavelength tunable laser sources TOS1 and TOS2 mayinclude, for example, a polymer wavelength tunable laser.

FIG. 5 is a view of the polymer wavelength tunable laser 10 used in awavelength tunable laser source according to an embodiment of thepresent invention.

The polymer wavelength tunable laser 10 outputs an optical signal of atunable wavelength. The polymer wavelength tunable laser 10 includes alaser diode 11 for outputting a Continuous Wave (CW) optical signal andwith an anti-reflection coating on a surface, a polymer Bragg-gratingwaveguide 14 for controlling a wavelength of a CW laser through externalresonance with the laser diode 11, and a heating electrode 12 forchanging and controlling a temperature of a Bragg grating by applyingheat to the polymer Bragg-grating waveguide 14.

The polymer Bragg-grating waveguide 14 is formed by fabricating awaveguide with a polymer material and forming a Bragg grating on thewaveguide, wherein a Bragg-grating waveguide is a passive optical devicethat reflects only an optical signal of an optical wavelength λ1determined by a grating interval from among optical signals of variousincident wavelengths and passes the remaining wavelengths.

Therefore, in an optical output of the laser diode 11 with ananti-reflection coating on a surface, the optical signal of the opticalwavelength λ1 is reflected by the polymer Bragg-grating waveguide 14 andreturns to the laser diode 11. Therefore, the laser diode 11 and thepolymer Bragg-grating waveguide 14 function as an external resonator,and as a result, the laser diode 11 outputs the optical signal of theoptical wavelength λ1.

Meanwhile, a polymer has a thermo-optic effect and has a characteristicthat a refractive index is changed by heat. Therefore, the polymerBragg-grating waveguide 14 may tune an optical wavelength reflected bythe heat applied by the heating electrode 12 to another opticalwavelength λ2, and accordingly a resonance wavelength between the laserdiode 11 and the polymer Bragg-grating waveguide 14 is tuned. As aresult, the laser diode 11 outputs an optical signal of the opticalwavelength λ2.

For example, the first and second wavelength tunable laser sources TOS1and TOS2 including the polymer wavelength tunable laser 10 output lightof a corresponding wavelength by wavelength control signals wc1 and wc2.

FIG. 6A is a view of a relationship between a wavelength control signaland an optical signal output from a wavelength tunable laser sourceaccording to the wavelength control signal.

Characteristically, the first and second wavelength tunable lasersources TOS1 and TOS2 in the present invention generate an opticalsignal in which a plurality of wavelengths appear alternately andperiodically, and optical power of the optical signal is constant or atleast continuous. An output optical signal of a wavelength tunable lasersource has a wavelength repetition period in which wavelengthsrepeatedly appear, and thus a wavelength control signal, which is anelectrical signal for controlling the wavelength tunable laser source,also has a period Tp.

Referring again to FIG. 4, the first wavelength tunable laser sourceTOS1 is controlled by the first wavelength control signal wc1, and thefirst wavelength control signal wc1 controls the first wavelengthtunable laser source TOS1 so that the first wavelength tunable lasersource TOS1 may generate the optical signal in which a plurality ofwavelengths appear alternately and periodically.

Furthermore, the second wavelength tunable laser source TOS2 iscontrolled by the second wavelength control signal wc2 which isidentical to the first wavelength control signal wc1 but has anadjustable delay time, and the second wavelength control signal wc2controls the second wavelength tunable laser source TOS2 so that thesecond wavelength tunable laser source TOS2 may generate the opticalsignal in which a plurality of wavelengths appear alternately andperiodically.

The first wavelength tunable laser source TOS1 generates a first opticalsignal in which a plurality of wavelengths appear alternately andperiodically, and the second wavelength tunable laser source TOS2generates a second optical signal which is identical to the firstoptical signal but has an adjustable delay time.

The delay unit DL outputs the second wavelength control signal wc2 bydelaying the first wavelength control signal wc1 by the delay time, andthe signal processing and controlling unit CONT controls the delay timeof the delay unit DL by a control signal d.

The first directional coupler DC1 emits an optical signal output fromthe first wavelength tunable laser source TOS1 to an optical line DUT tobe measured and transmits a portion of light returning from a cutsurface of the optical line DUT to a polarization controller PC of theinterferometer IFM.

The interferometer IFM receives a reflected optical signal,corresponding to the first optical signal, which is returning afterhaving been emitted to the optical line DUT, and the second opticalsignal, and causes interference between the reflected optical signal andthe second optical signal to output an interference signal.

The interferometer IFM has two input ports and one output port, and oneof the two input ports receives the reflected optical signal from thefirst directional coupler DC1 and the other one receives the secondoptical signal from the second wavelength tunable laser source TOS2.

Generally, the two optical signals have different polarizations. It is ageneral structure of an interferometer to place the polarizationcontroller PC in one of the two ports because no interference occurs ifthe polarizations do not match each other.

The interferometer IFM includes a second directional coupler DC2 and apolarization controller PC, wherein the polarization controller PC isfor matching the polarizations of the reflected optical signal and thesecond optical signal, and the second directional coupler DC2 transmitsan optical signal from the polarization controller PC and the secondoptical signal to the optical signal receiver PD.

The optical signal receiver PD receives an optical signal output fromthe interferometer IFM and converts the optical signal into anelectrical signal, and the analog-to-digital converter ADC converts ananalog electrical signal into a digital electrical signal.

The signal processing and controlling unit CONT manages an operation ofthe entire optical line testing device 100, particularly provides acontrol signal for controlling a wavelength tunable laser source and adelay unit, receives a digital electrical signal from theanalog-to-digital converter ADC to perform signal processing, andcalculates at what point an optical line is cut.

Hereinafter, an operation of an optical line testing device of thepresent invention will be described with reference to FIGS.

The signal processing and controlling unit CONT applies the firstwavelength control signal wc1 to the first wavelength tunable lasersource TOS1. The first wavelength tunable laser source TOS1 outputs anoptical signal in which output wavelengths change alternately accordingto a control signal as shown in FIG. 6A, and the optical signal isincident on the optical line DUT to be measured.

Thereafter, light reflection occurs at a cut surface of the optical lineDUT, and a portion of the optical signal sent from the first wavelengthtunable laser source TOS1 is reflected back to an original position andproceeds backwards. A portion of the proceeding optical signal is inputto the interferometer IFM by the first directional coupler DC1.

The interferometer IFM has two input ports and one output port, whereinone of the two input ports receives an optical signal from the firstdirectional coupler DC1 and the other one receives an optical signalfrom the second wavelength tunable laser source TOS2, and thepolarization controller PC controls and matches polarizations of the twooptical signals.

An output of the interferometer IFM is input to the optical signalreceiver PD and an output electrical signal of the optical signalreceiver PD is input to the signal processing and controlling unit CONTvia the analog-to-digital converter ADC, wherein the output electricalsignal is used to determine a position of a cut surface in an opticalline.

Hereinafter, a principle of measurement using an optical line testingdevice according to an embodiment of the present invention will bedescribed step by step starting from an assumption.

First, it is assumed that a length of the optical line DUT to bemeasured is zero and a length of all internal optical lines in theoptical line testing device 100 is also zero.

With this assumption, a case where a delay time Td between the firstwavelength control signal wc1 applied to the first wavelength tunablelaser source TOS1 and the second wavelength control signal wc2 appliedto the second wavelength tunable laser source TOS2 is set to 0 will bedescribed. For example, a shape of the two wavelength control signals isas shown in FIG. 6B.

In this case, there is no delay time between the signals of the firstwavelength tunable laser source TOS1 and the second wavelength tunablelaser source TOS2, and thus an interference effect between the twooptical signals reaching the interferometer IFM is maximized and anoutput of the optical signal receiver PD is maximized (the output isreferred to as PD_max).

A case where the delay time Td is applied between the first wavelengthcontrol signal wc1 applied to the first wavelength tunable laser sourceTOS1 and the second wavelength control signal wc2 applied to the secondwavelength tunable laser source TOS2 will be described in FIG. 6C.

In this case, since a frequency difference between the signals of thefirst wavelength tunable laser source TOS1 and the second wavelengthtunable laser source TOS2 is the greatest, an output of the opticalsignal receiver PD becomes minimum (the output is referred to asPD_min).

FIG. 7A is a graph of an output of the optical signal receiver PDaccording to the delay time Td when it is assumed that a length of theoptical line DUT to be measured is zero and a length of all internaloptical lines in the optical line testing device 100 is also zero.

As shown in FIG. 7A, when an output of the optical signal receiver PD isdisplayed in a graph while varying the delay time Td, a repetitivepattern in which the period Tp of a wavelength control signal is onecycle appears (a horizontal axis in the graph is the delay time Td).

It is determined how the output of the optical signal receiver PD varieswhen there is a length of the optical line DUT to be measured (an actuallength of the optical line DUT to be measured is not 0 but L(m)). It isassumed that a round trip time when light travels to the end of anoptical line of length L(m) and then reflects back is 2Tr.

First, an output of the optical signal receiver PD will be displayed inthe graph while gradually increasing the delay time Td in a state wherethe delay time Td is zero as shown in FIG. 7B. Then, the delay time Tdat a point where the output becomes maximum in the graph will be 2Trdescribed above. This is because a signal of an interferometer becomesmaximum when the round trip time 2Tr via the optical line DUT to bemeasured is equal to the delay time Td of a signal transmitted to thesecond wavelength tunable laser source TOS2.

In this way, the light beam tester 100 may determine a value of theround trip time 2Tr, and may calculate that a cut surface of an opticalline is located at a distance L from the value.

The optical line testing device 100 according to an embodiment of thepresent invention measures an output of an interference signal whilevarying the delay time Td, wherein the interferometer IFM is used tooutput the interference signal by causing interference between areflected optical signal, corresponding to a first optical signal, whichis returning after having been emitted to an optical line, and a secondoptical signal to which a delay time is given. Then, a cutting positionof the optical line is calculated using the delay time at which themeasured output becomes maximum.

The first optical signal generated from the first wavelength tunablelaser source TOS1, which is one of two wavelength tunable laser sources,is emitted to the optical line, and the variable delay time Td is givento the second optical signal generated from the second wavelengthtunable laser source TOS2, which is the other one of the two wavelengthtunable laser sources.

According to an embodiment of the present invention, the cuttingposition is measured by using an interference effect when the round triptime 2Tr, corresponding to the first optical signal, which is returningafter having been emitted to the optical line, is equal to or anintegral multiple of the delay time Td of the second optical signal.

Meanwhile, it is possible to determine a position of the cut surface ofthe optical line if the period Tp of the wavelength control signal isincreased, but it is difficult to determine the exact position. This isbecause a maximum point of PD_max is not clearly distinguished as shownin FIG. 8A.

In order to solve the problem, in the other embodiment of the presentinvention, a plurality of wavelength control signals having differentperiods are used to measure the cutting position.

FIGS. 9A to 9C are views of a plurality of wavelength control signalshaving different periods and an output of a wavelength tunable lasersource according to each of the plurality of wavelength control signals,wherein FIG. 9A shows a case where the period Tp of the wavelengthcontrol signal is T1, FIG. 9B shows a case where the period Tp of thewavelength control signal is T2, and FIG. 9C shows a case where theperiod Tp of the wavelength control signal is T3 (T1≠T2≠T3).

FIGS. 9A to 9C may be the same as FIG. 8B when the optical line testingdevice 100 sequentially measures an output of an interference signalwhile varying the period Tp of the wavelength control signal, anddisplays outputs of the optical signal receiver PD superimposed on agraph.

According to the other embodiment of the present invention, accurateposition information of an optical line is determined by superimposinginterference results obtained by using wavelength control signals havingdifferent periods.

The period Tp of the wavelength control signal becomes equal to a periodin which any one wavelength appears repeatedly in an optical signaloutput from a wavelength tunable laser source (hereinafter, referred toas a ‘wavelength repetition period’).

According to the other embodiment of the present invention, an output ofan interference signal is measured while varying the delay time Td fortwo or more different wavelength repetition periods, and in all of thetwo or more different wavelength repetition periods, a cutting positionis calculated using a delay time at which the measured output becomesmaximum.

Hereinafter, effects according to configurations of the presentinvention will be described.

According to a conventional optical line testing device, it isimpossible to increase a magnitude of an optical pulse by more than acertain limit by using a method of detecting a cutting position of anoptical line using the optical pulse in which optical power changesabruptly. If the magnitude of the optical pulse is greater than acertain limit, a nonlinear effect occurs between the optical line andthe optical pulse, so that a measurement error occurs, and if a width ofthe optical pulse is increased, the resolution is deteriorated.

Meanwhile, according to an embodiment of the present invention, opticalpower of a used optical signal is constant or continuous, so that evenif the optical power increases, a nonlinear effect between the opticalsignal and an optical line does not occur or decreases.

Therefore, according to an embodiment of the present invention, even ifoptical power of an optical signal for measurement is increased, adynamic range may be greatly improved without sacrificing a resolutionof an optical line testing device.

According to an embodiment of the present invention, it is possible toprovide an optical line testing device with excellent resolution anddynamic range, and to precisely measure a cutting position or the likeon a long-distance optical line.

Furthermore, according to an embodiment of the present invention, it ispossible to minimize a nonlinear effect caused by a measured opticalsignal in an optical line so that an optical amplifier such as an ErbiumDoped Fiber Amplifier (EDFA) may be used.

What is claimed is:
 1. An optical line testing device comprising a firstoptical source configured to generate a first optical signal having acontinuous waveform, and output the first optical signal to an opticalline, wherein the first optical signal is output to the optical linewhile maintaining a shape of the continuous waveform; a second opticalsource configured to generate a second optical signal, an interferometerconfigured to receive a reflected optical signal, that is a signal ofwhich the first optical signal is reflected from the optical line, andthe second optical signal, and output an interference optical signalgenerated by an interference between the reflected optical signal andthe second optical signal; and a processor configured to adjust a timedelay between the first and second optical signals, and measure a faultlocation of the optical line based on the time delay and theinterference optical signal.
 2. The optical line testing device of claim1, wherein each of the first and second optical signals is an opticalsignal having a periodic continuous waveform.
 3. The optical linetesting device of claim 2, wherein, the first and second optical signalshave same wavelength and same amplitude.
 4. The optical line testingdevice of claim 2, wherein each of the first and second optical signalsis an optical signal in which a plurality of wavelengths appearalternately and repeatedly.
 5. The optical line testing device of claim2, wherein the processor is configured to calculate the fault locationby using a time delay at which the interference optical signal becomesmaximum.
 6. The optical line testing device of claim 1, wherein theinterferometer includes a polarization controller for matchingpolarizations of the reflected optical signal and the second opticalsignal.
 7. The optical line testing device of claim 1, furthercomprising: a converter configured to convert the interference opticalsignal into an electrical signal.
 8. An optical line testing devicecomprising: a first optical source configured to generate a firstoptical signal having a continuous waveform, and output the firstoptical signal to an optical line in response to a first control signal,wherein the first optical signal is output to the optical line whilemaintaining a shape of the continuous waveform; a second optical sourceconfigured to generate a second optical signal in response to a secondcontrol signal; an interferometer configured to receive a reflectedoptical signal, that is a signal of which the first optical signal isreflected from the optical line, and the second optical signal, andoutput an interference optical signal generated by an interferencebetween the reflected optical signal and the second optical signal; anda processor configured to adjust a time delay between the first andsecond optical signals by controlling a time delay between the first andsecond control signals, and measure a fault location of the optical linebased on at least one of the time delays and the interference opticalsignal.
 9. The optical line testing device of claim 8, wherein the timedelays are identical to each other.
 10. The optical line testing deviceof claim 8, wherein each of the first and second control signals is asignal having a periodic pulse waveform.
 11. The optical line testingdevice of claim 10, wherein the first and second control signals havesame pulse width and same amplitude.
 12. The optical line testing deviceof claim 8, wherein each of the first and the second optical signals isan optical signal having a periodic continuous waveform.
 13. The opticalline testing device of claim 12, wherein the first and second opticalsignals have same wavelength and same amplitude.
 14. The optical linetesting device of claim 12, wherein each of the first, and secondoptical signals is an optical signal in which a plurality of wavelengthsappear alternately and repeatedly.
 15. The optical line testing deviceof claim 12, wherein the processor is configured to calculate the faultlocation by using a time delay at which the interference optical signalbecomes maximum.
 16. The optical line testing device of claim 8, furthercomprising a buffer configured to generate the second control signal bydelaying the first control signal by the time delay.
 17. The opticalline testing device of claim 8, wherein the interferometer includes apolarization controller for matching polarizations of the reflectedoptical signal and the second optical signal.
 18. The optical linetesting device of claim 8, further comprising: a converter configured toconvert the interference optical signal into an electrical signal.